Spamassassin is more difficult to configure because commercial products don't have the luxury of requiring more sysadmin configuration. They have to be easy or no one would buy them. The disadvantage of them being easier is that they have less flexibility, less information and less site-specific configuration to work with. They also tend to be less accurate, erring to the side of enforcement at the risk of discarding legitimate mail.

It is important to check spamassassin to see which plugins are installed properly and working. Spamassassin will work with only a few plugins installed, but it will work much better if you install all plugins that make sense for your site.

To maintain spamassassin well, you also have to have very level-headed admins who are willing to drop even very effective plugins if they have the potential for false positives. You have to evaluate the plugins yourself, to some extent, and you have to trust behavior that you observe. I recently had to decrease the score of the BOTNET plugin significantly. It's not the BOTNET plugin is doing something wrong -- it's simply that companies often configure their mail servers with mail gateways and have internal/private network Received lines that trigger the BOTNET plugin.

Commercial products tend to trap lots of spam, like a properly configured spamassassin installation, but they also tend to get a lot of false positives. Consider that people complain a lot more about false negatives (spam that gets through) than false positives, especially if they don't see the false positives. Because of this behavior pattern, commercial products will almost always err to the side of throwing away the baby with the bathwater. And this is more dangerous to email than spam is.

Best,
Jesse

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